Carburetor evaluation system

ABSTRACT

This disclosure deals with a system for evaluating the performance of a carburetor of an internal combustion engine while the engine is operating. The system measures the mass flow rate and temperature of the air entering the carburetor, the mass flow rate and temperature of the fuel entering the carburetor, the fuel level in the float bowl of the carburetor, and the engine speed. These parameters, and/or combinations thereof, are recorded under different engine operating conditions, in order that the recorded data may be later compared with the performance of a &#39;&#39;&#39;&#39;standard&#39;&#39;&#39;&#39; carburetor.

United States Patent Vanderbilt, Jr. et al. [451 Sept. 19, 1972 [5CARBURETOR EVALUATION SYSTEM 3,439,534 4/1969 Pilgrim ..73/l i7 72]Inventors: vem C. Vanderbilt Jr. 3,517,552 6/1970 Converse et al ..73/ll8 Hagerstown; Chrence L. Zimmer, 3,525,969 8/1970 Newman ..73/398 ARXRichmond; William F. Van Ostrand; Gerald Mamas, both of PrimaryExaminer-Jerry W. Myracle Raga-Stow, a f Attorney-Hibben, Noyes &Bicknell [73] Assignee: Dynamic Precision Controls Cor- [57] ABSTRACTporation, l-lagerstown, Ind. TH d l d al h f I h IS isc osure e s wit asystem or eva uatmg t e [22]. led: June 1970 performance of a carburetorof an internal com- 2 1 ;43, g bustion engine while the engine isoperating. The system measures the mass flow rate and temperature of theair entering the carburetor, the mass flow rate [2%] 1.1.8.5: ..73/1l8,331/64 and temperature of-the fuel entering the carburetor 8] Fntidl?/00 the fuel level in the float bowl of the carburetor, and [5 re oare I118, 33 [64 the engine Spemi These parameters, and/or combina 56] Rf d tions thereof, are recorded under different engine I e erencesoperating conditions, in order that the recorded data UNITED STATESPATENTS may be later compared with the performance of a standard"carburetor. 1,895,047 l/l933 Neumann ..73/2l3-UX 1,898,951 2/1933Goodwin ..73/2ll 14 Claims, 11 Drawing Figures 34 38 39 RANGE 31 ANS-SELEITOR E 35 DUCER c DNTROL iqt' n hn ig? 55 5g DISPLAY- COUNTERPRINTER CONTROL UNIT PATENTED SEP 1 9 m2 SHEET 2 BF 5 PATENTED SEP 19I972 SHEET 3 BF 5 PATENTED SEP 19 I972 SHEET ll 0F 5 1 CARBURETOREVALUATION SYSTEM evaluating the performance of a carburetor of such anengine to determine whether it meets indusn'y standards, and it is notcapable of combining certain parameters to derive, for example, ratiosof them. Information of this character is important in evaluating acarburetor because an improperly operating carburetor results in poorengine performance and an increase in pollutants produced by the engine.

In accordance with the present invention, a carburetor evaluation systemis provided, comprising means adapted to be connected to the air intakeof a carburetor for measuring the mass of air per unit of time flowin ginto the carburetor, means adapted to be connected to the fuel intake ofthe carburetor for measuring the mass of fuel per unit of time flowinginto the carburetor, and means adapted to be connected to the engine formeasuring engine speed. There is preferably also provided means adaptedto be connected to the carbu retor float bowl for sensing the fuel leveltherein. The system further includes means for combining electricalsignals representing the air and fuel mass flow rates to derive signalsrepresenting the air-fuel ratio at selected engine speeds. A device ispreferably provided to make a record of the various signals.

Further objects and advantages of the invention will become apparentfrom the following description taken in conjunction with theaccompanying figires of the drawings, in which:

FIG. 1 is a view of an automobile equipped with a carburetor evaluationsystem embodying the invention;

FIG. 2 is a schematic illustrafion of the system;

FIG. 3 is a sectional view showing a portion of the system for measuringthe mass flow rate of fuel;

FIG. 4 is a sectional view taken on the line 4-4 of FIG. 3;

FIG. 5 is a fragmentary sectional view taken on the line 5-5 of FIG. 3;

FIG. 6 is a view showing a portion of the system for measuring the massflow rate of air;

FIG. 7 is a fragmentary enlarged sectional view of a portion of thestructure shown in FIG. 6;

FIG. 8 is a reduced view taken on the line 8-8 of FIG. 7;

FIG. 9 is a view of an electrical circuit of the system;

FIG. 10 is a fragmentary view showing a portion of the system formeasuring fuel level; and

FIG. 11 is a block diagram of a computer forming part of the system.

In FIG. 1 is illustrated the fuel supply system of an automobile 15including an internal combustion engine 16, the fuel supply systemincluding a fuel storage tank 17, a fuel pump 18, and a carburetor 19. Afuel line 21 connects the fuel tank 17 to the intake of the fuel pump18, and another line 22 conducts fuel away from the outlet or highpressure side of the fuel pump 18. Normally, the line 22 has its outletconnected directly to a float bowl of the carburetor 19, but in thepresent instance, it is connected to apparatus 24 of a system designedto evaluate the operation of the carburetor 19. In the present instance,the apparatus 24 is mounted on top of the air intake of the carburetor19. Another fuel line 26 connects the apparatus 24 with the carburetor19, thus completing the fuel flow circuit from the fuel pump 18, throughthe structure 24 which measures the mass per unit of time of fuelflowing to the carburetor 19. As will be described in greater detailhereinafter, the air drawn into the carburetor 19 also flows through theapparatus 24, and the apparatus 24 is constructed to measure the massper unit of time of air flowing therethrough. The apparatus 24 is alsodesigned to sense the fuel level in the carburetor float bowl, and theengine speed. Electrical signals representing the foregoing parametersof the carburetor l9 and the engine 16 are fed through a multipleconductor cable 27 to an instrument 28 which includes driver-operatedcontrols and a recorder.

In FIG. 2, the carburetor 19 and the system parts connected thereto arerepresented schematically. Ahead of the air intake of the carburetor 19is a multiple venturi arrangement indicated generally by the number 31,and ahead of the fuel intake to the carburetor is an oriface arrangementindicated generally by the numeral 32. The portion of the system formeasuring the mass per unit of time of air flowing into the carburetor19 further includes a temperature sensor 33, such as a therrnister,positioned adjacent the entrance of the venturi 31, and pressure sensors34 and 35.'The sensor 34, which may be an aneroid barometer, measuresbarometric pressure, and the sensor 35 measures the pressuredifferential between the entrance and the throat of the venturi 31. Thesensors 33, 34 and 35 are connected to a transducer 38 which generatesan electrical signal on a conductor 39, the frequency of the signal onthe conductor 39 being linearly related to the mass of air per unit oftime flowing through the venturi 31 and into the carburetor 19.

The portion of the system for measuring the mass flow rate of fuelcomprises the orifice 32, a temperature sensor 42 positioned in the line22 to measure the temperature of the fuel flowing therethrough, and apressure differential sensor 43 connected to measure the fuel pressuredifference on opposite sides of the orifice 32. The sensors 42 and 43are connected to another transducer 44 which generates a signal on aconductor 46 having a frequency which is linearly related to the mass offuel per unit of time flowing into the carburetor 19.

The system further includes a probe 48 for sensing the level of the fuelin the float bowl (FIG. 10) of the carburetor. As will be described ingreater detail hereinafter, the probe 48 includes an electrical heatingcoil 51 and a pair of temperature-sensitive elements 52 and 53, such asthermisters. In the arrangement shown in FIG. 2, the sensors 52 and 53are connected to the transducers 38 and 44, respectively.

The system further includes a unit 56 for sensing the speed of theengine 16. In the present instance, the unit 56 includes a toothed wheelor gear 57 fastened to a shaft 58 which is connected to be rotated bythe crankshafi of the engine at a rate which is linearly related to thespeed of the engine. An inductance pick-up comprising a coil 59 and apermanent magnet 61 is located adjacent the wheel 57, and the coil 59 isconnected to a conductor 62. Rotation of the toothed wheel 57 by theengine 16 results in electrical pulses being generated and transmittedby the conductor 62, the frequency of the pulses being linearly relatedto the rate of rotation of the wheel 57 and the engine 16.

- The signals on the conductors 39, 46 and 62, as well as any additionalinput signals on other conductors 63 and 64 which may be connected tosense other operat- V ing parameters of the engine, arev connected to acontrol unit 66 of a computer-counter (FIG. 1 1) located in the controlinstrument 28 (FIG. 1). When an operator wishes totake a reading, heinserts alpunched card 67 (FIG. 2) into the computer-counter. Thepunched instructions programmed on the card 67 are read and operate theremainder of the units in the instrument 28 as-will be latter describedin connection with FIG. 11. The remaining units of the computer-counterinclude a counter 68 which provides a signal on a conductor 69 to adisplay-printer unit 71 which displays and/or prints the information. Anelectronic frequency responsive control 73 is preferably also providedto enable the printer 71 to operate only when the engine is operating ina selected range.

FIGS. 3 through illustrate the orifice and transducer for measuring themass of fuel flowing into the carburetor. This structure operates on theprinciple that the pressure drop through an orifice through which thefuel flows is related to the rate at which fuel flows through theorifice..This structure comprises a hollow, generally rectangularenclosure 91 having a hollow member 92 secured within the enclosed space90 thereof. The axis of the member 92 extends generally vertically andit is located at approximately the center of the enclosure 91. The lowerend of the member 92 is secured to a flat base 93 having a fuel intakepassage 94 formed therein, the base 93 being secured to the bottom wallof the enclosure 91. As shown in FIG. 4, a tube 96 extends from thepassage 94 and through a hole 97 formed in a wall of the enclosure 91and the fuel line 22 leading from the fuel pump 18 (FIG. 1) is connectedto the tube 96. Thus, fuel from the pump 18 flows through the tube 96,through the passage 94 formed in the base 93, and into the hollowinterior 98 of the member 92.

The upper end of the member 92 is closed by a cylindrical block 99 whichis sealingly secured within the upper end of the member 92. verticallyextending orifice 101 is formed in a screw 102 which is threaded in ahole 103 formed through the block 99. Thus, fuel flowing into theinterior 98 of the member 92 flows upwardly through the orifice 101 andinto the space 90 formed by the enclosure 91 and externally of themember 92. A fitting 104 (FIG. 4) is fastened in an opening 105 formedthrough the top wall of the enclosure 91, the fitting 104 being adaptedto be connected to the line 26 which leads from the structure 24 to theintake of the carburetor 19.

During operation of the engine, the fuel in the interior 98 of themember 92 is at a higher pressure than the fuel in the enclosure 90.This is true because consumption of fuel by the engine causes fuel flowout of the enclosure 90 with a resultant pressure drop in the enclosure90 whereas the pressure in the interior 98 of the member 92 issubstantially at the pressure of the fuel delivered by the pump 18 whichis nearly constant. The pressure differential between the interior 98and the enclosure 90 is related to the mass of fuel per unit of timeflowing through the orifice 101. To measure this pressure difl'erence, acommercially available bellows 111 is provided, having its interiorconnected by a passage 112 to sense the fuel pressure within theinterior 98 and its exterior located to sense the pressure in theenclosure 90. A frame 113 supports thebellows 111 on a wall of themember 92, and the exterior surface of the bellows 111 is exposed to thefuel within the enclosure 90. The frame 113 also supports a variableresistor 114 having a 'movable wiper 116 which is attached to themovable outer end of the bellows 1 1 1.. A conductor 117 connects thevariable resistorl 14 inthe circuit of the transducer 44, which will bedescribed in greater detail hereinafter.

' When the engine is operating at idling speeds,the

fuel pressure within the enclosure 90 around the exterior of the bellows111 will be nearly that of the pressure within the bellows because fuelconsumption is relatively low. However, when the engine speeds up andconsumes a greater amount of fuel, the pressure within the enclosure 90drops and is less than the pressure within the interior of the bellows.The bellows 1 1 1 then expands and the variable resistor 114 reflectsthe, expansion. 7

When the pressure difference reaches a'certain level, the bellowsreaches the limit of its normal expansion, and thereafter there isdanger'that damage may occur I to the bellows 111 and the variableresistor 114. Such damage is prevented, however, by providing a secondorifice 121 formed in a screw 122 located adjacent the first-mentionedorifice 101. Also provided is' a relief valve arrangement opening thesecond orifice 121 only at high pressure differences. The orifice 121'isclosed at low pressures by a resilient disc-shaped seal 123 of thearrangement 120, which is held against the upper surface of the member99 and covers the orifice 121. The seal 123 is supported by an L-shapedlatch arm 124 which is pivotally fastened by a pin 126 (FIGS. 3 and 5)to a pair of L-shaped supports 127 and 128. The supports 127 and 128 arefastened to the member 99 by screws 140. The latch arm 124 includes ahorizontal section which extends over the member 92, and a verticalsection which extends adjacent a vertical side of the member 92. The endof the horizontal section of the arm 124 is pivotally connected to alink 131 by another pin 132. As shown in FIG. 3, the link 131 supports ascrew 133 to which is fastened a metal disc 134, and the seal 123 issecured on the lower side of the disc 134. The screw 133 may be turnedto adjust the vertical position of the seal 123. The vertical section ofthe arm 124 has a flat plate 136, which is made of a magnetic material,secured to the free end thereof. The lower ends of the two supports 127and 128 extend downwardly generally parallel to the arm 124, and apermanent magnet 137 is secured between the lower ends thereof. Themagnet 137 is held in a channelshaped member 138 by a screw 139, and themember 138 is secured to the lower ends of the supports 127 and 128 byadditional screws 141. The'plate 136 is secured to the lower end of thearm 124 by screws 142. The magnet 137 is preferably recessed in themember 138 so that it is not engaged by the plate 136. Further, theplate 136 is preferably prevented from being too firmly held by themagnet 137, and this may be accomplished as by plating the member 138with a non-magnetic metal.

When the assembly is in its latched position, shown in full lines inFIG. 3, the plate 136 is held by the magnet 137 and the resilient seal123 covers the orifice 121. A compression spring 143 is positioned in ahole 144 between the horizontally extending section of the arm 124 andthe upper surface of the member 99, and tends to lift the seal 123 011the member 99 and thus open the orifice 121. However, at low enginespeeds the force of the magnet 137 is sufficiently strong to overcomethe force of the spring 143 and the force of the fuel acting through theorifice 121. An adjusting screw 145 is preferably fastened to the arm124 above the spring 143 in order to adjust the force of the spn'ng whenthe assembly is latched. As engine speed increases and the fuel pressurewithin the enclosure 90 drops, the fuel pressure acting through theorifice 121 against the underside of the seal 123, combined with theforce of the spring 143 is sufficient at a certain speed to overcome theforce of the magnet 137. The arm 124 then moves to its tripped positionshown in dashed lines in FIG. 3 wherein the plate 136 is displaced fromthe magnet 137 and the seal 123 is lifted off the member 99. Thereafter,fuel flows from the interior 98 through both of the orifices 101 and121. The size of each orifice will depend, of course, upon the size ofthe engine. As an example, the diameter of the small orifice 101 may beapproximately 0.043 inch and the diameter of the large orifice 121 maybe approximately 0.065 inch.

Movement of the arm 124 to its tripped position accomplishes twofunctions. First of all, it prevents damage to the bellows 11 1 becauseit is adjusted to trip before the pressure is great enough to damage thebellows. Secondly, it extends the range of the fuel sensing apparatus byincreasing the effective size of the orifice through which the fuelflows. Consequently, a single bellows transducer is capable of sensingthe mass flow of fuel over a wide range of fuel flow rates.

After the arm 124 has moved to the tripped position, it is necessary toreturn it to its latched position before the system may be used again atlow engine speeds. To return the arm 124 to the latched position, amanually operable push-button type of device is provided, comprising apin 146 which extends through a sealed opening 147 formed in the topwall of the enclosure 91 above the horizontally extending section of thearm 124. A flat disc 148 is secured to the upper end of the pin 146, anda spring 149 is positioned between the disc 148 and the enclosure 91 tourge the pin 146 upwardly. A washer 151 is secured to the pin 146 withinthe enclosure 91 to prevent the pin from coming out of the hole 147.When the operator presses downwardly on the disc 148, the pin 146 ismoved downwardly and its lower end engages the arm 124 and moves it tothe point where the force of the magnet 137 pulls the arm 124 to itslatched position. A solenoid actuated device could, of course, beprovided in place of the manually operated pin 146 and the disc 148 toreturn the arm 124 to its latched position.

As shown in FIGS. 3 and 4, the fuel temperature sensor 42 may besuspended from the top wall of the member 91 within the enclosure 90.

With reference to FIGS. 6 to 8, the venturi arrangement 31 (FIG. 2) formeasuring the mass of air per unit of time flowing into the carburetor19 comprises, in the present instance, three concentric air horns 160,161 and 162 which are of progressively decreasing size. The medium-sizedair horn 161 is mounted within the largest air horn and is supportedtherein by three radially extending struts 163. The largest air horn 160includes a constriction or venturi indicated by the reference numeral164, and the downstream end of the medium-sized air both 161 terminatesat approximately the entrance of the venturi 164. The medium-sized airhorn 161 also includes a venturi 166, and the downstream end of thesmall-sized air horn 162 terminates at the entrance of the venturi 166.The air horn 162 includes a venturi 168 and is also supportedconcentrically within the large-sized air horn 160 by a plurality ofradially extending struts 167.

To sense the pressure within each venturi while air is flowingtherethrough, a conduit or tube 171 is positioned in a hole 172 throughthe wall of the large air horn 160, the inner end of the tube 171opening into the venturi 164. Another tube 173 extends through a hole174 in the medium-sized air horn 161, and the inner end of the tube 173opens into the venturi 166. The tube 173 also extends outwardly througha hole 176 through the wall of the horn 160. The pressure within theventuri 168 of the small horn 162 is sensed by a tube 177, the inner endof the tube 177 extending through a hole 178 in the wall of the horn 162and opening in the venturi 168. The tube 177 also extends outwardlythrough a hole 179 in the wall of the large horn 160. The pressure atthe downstream end of the large horn 160 is sensed by a tube 183 whichis positioned in a hole 184 in the wall of the horn 160.

As is well known, the amount of air flowing through a venturi may bedetermined if the venturi size is known and if the difference betweenthe air pressure at the entrance to the venturi and at the throat of theventuri is known. The pressure at the entrance to the air horn is sensedby a tube 181 located in a hole 182 through the wall of the large airhorn 160 at the entrance thereof, and as will be discussed hereinafter,the pressure in the tube 181 is compared with the pressures in theventuris as sensed by the tubes 171, 173 and 177.

With reference to FIG. 6, the tubes 171, 173, 177 and 183 are connectedto a switching device 186 for selectively connecting one of the tubes171, 173, 177 or 183 to an outlet tube 187 which leads to the sensor 35.The switch 86 may be manually operated or it may be automaticallyoperated in response to signals received from a card reader to bedescribed in connection with FIG. 11. The tube 181 which senses the highpressure at the upstream end of the air horn 160 is connected directlyto another input of the sensor 35. The transducer 35 senses thedifference in pressure between the high pressure tube 181 and one of thefour tubes 171, 173, 177 and 183, depending upon the position of theswitch 186.

The transducer 38 preferably includes a bellows 188 and a variableresistor 189 generally similar to the bellows 111 and variable resistor114 arrangement shown 7 in FIG. 3. The main difference between thebellows 188 for the air flow and the bellows 11 1 for the fuel flow isthat the former is larger due to the relatively small pressure dropencountered in a venturi as compared with the orifice. The high pressuretube 181 may be connected to the inside of the bellows and the tube 187may be connected to the outside. The variable resistor 189 is connectedin an oscillator circuit to be described, which produces a variablefrequency signal on the conductor 39.

When air flows through the horn 160 to the carburetor, the air pressureat the tube 181 will be greater than the pressure at any of the tubes177, 173, 171 and 183. Since the tube 183 does not open at the throat ofa venturi, the pressure difference between the tubes 181 and 183 will bedue to frictional losses resulting from air flow through the horn. Thepressure difference between the tubes 181 and 171 is due to the usualventuri effect. The pressure difference between the tubes 181 and 173 isalso due to the venturi effect. Since the downstream end of the horn 161terminates in the venturi 164, the pressure drop due to the venturi 164appears at the outlet of the horn 161, thus causing increased air flowthrough the horn 160 and pressure drop in the venturi 166. Similarly,the pressure drop in the venturi 168 of the air horn 162 is relativelylarge because its downstream end terminates in the throat 166 of thehorn 161, and the pressure drops in the venturi 164 and in the venturi166 multiply the effectiveness of the venturi 168. For a given air flowrate the pressure difference between the tubes 181 and 177 will begreatest; that between the tubes 181 and 173 will be the next greatest,that between the tubes 181 and 171 will be next greatest, and thatbetween the tubes 181 and 183 will be the least. Consequently, when theengine is operating at very low speeds, the switch 186 is adjusted toconnect the tube 177 only to the transducer 38, and as the engine speedsup the operator turns the switch 186 to connect the tubes 173, 171 and183 in succession to the transducer 38. Each of the tubes 177, 173, 171and 183 is thus used in a different range of air flow and in each rangeof air flow the pressure differential is approximately the same.Consequently, the signal out of the transducer 38 sweeps throughapproximately the same range of frequencies. Thus, the structure shownin FIGS. 6 to 8 is advantageous because only a single transducer isrequired to measure a wide range of air flows.

As previously mentioned, the system includes a plurality of transducers,such as the transducers 38 and 44, which sense various operatingparameters of the carburetor and the engine and generate electricalsignals having frequencies which are functions of the parameters. Eachtransducer includes an oscillator circuit of the character shown in FIG.9, this oscillator circuit being an improvement over the circuit shownin FIG. 3 of Vern. C. Vanderbilt US. Pat. No. 3,308,360. FIG, 3 of theabove patent shows an oscillator circuit having a notch tuning networkincluding a pair of potentiometers, the wipers of the potentiometersbeing connected together and to a movable member which is responsive toa parameter being sensed- The frequency of the oscillator is linearlyrelated to variations in the resistances of the two potentiometers.While the oscillator circuit shown in the above patent works well, it isconstructed to sense only one parameter at a time.

The circuit shown in FIG. 9 of the present application is capable ofresponding simultaneously to two or more parameters. The presentoscillator circuit includes an amplifier 201 having an output connection202 and a differential input arrangement including a positive input 203and a negative input 2030. An input signal applied to the positive input203 produces an output changing in a direction'the same as (or in phasewith) that input. An input signal applied to the negative input 203aproduces an output changing in a direction opposite to (or out of phasewith) the input. A buffer amplifier (not shown) is preferably connectedto the output 202 of the amplifier 201. A conductor leads from theoutput 202 to an amplitude stabilization lamp 221, a capacitor 206 andto the positive input 203 thus forming a positive feedback loop. Aresistor 207 connected between ground and the input 203 provides a loadfor the amplitude stabilization lamp 221. While some method of amplitudestabilization is necessary, a method other than a lamp may be used. Anegative feedback loop includes a notch tuning circuit connected to theinput 203a of the amplifier 201, comprising a pair of series-connectedpotentiometers 208 and 209, the junction of the two potentiometers 208and 209 being connected to ground potential through a coil 211 and acoupling capacitor 212. The resistor 209 is connected to a resistancenetwork 210 including two serially connected resistors 213 and 214, andtwoadditional serially connected resistors 215 and 216. The tworesistors 215 and 216 are connected in parallel with the two resistors213 and 214, and the juncture of the two resistors 214 and 216 isconnected to ground through a coupling capacitor 219. The juncture ofthe two resistors 213 and 215 is connected to the feedback conductor204. The juncture of the two resistors 213 and 214 is connected to thevariable resistor 209, and the juncture of the two resistors 215 and 216is connected through a coil 222 to the negative input 203a of theamplifier 201.

The oscillator drive signal from the output of the amplifier 201 appearson the conductor 204 and is fed through the positive feedback loop,including the lamp 221 and the capacitor 206, to the amplifier positiveinput 203. The drive signal is also connected to the drive terminal 223of the negative feedback loop including the notch circuit. At the notchfrequency, which is the frequency to which the notch circuit is tuned, apositive feedback signal appears at the positive input 203 of theamplifier, causing the oscillator to oscillate. The frequency at whichoscillation occurs may be changed by varying the values of theparameters of the notch circuit.

Considering firstthe operation when the resistances of the resistancenetwork 210 are held constant, if the resistances of the two variableresistors 208 and 209 were simultaneously increased, the frequency ofoscillation would increase. Further, the frequency will vary as a directfunction of changes in resistances of the two potentiometers 208 and 209when the two potentiometers are equal and the amounts of increase areequal. If only one of the two potentiometers 208 and 209 were increased,the frequency of oscillation would increase as the square root of theincrease of resistance.

Considering the influence of changes in the resistance network 210 onthe operation of the oscillator circuit, it is an important feature ofthe present invention that a change in the resistance of one of theresistors of the resistance network 210 will also change the frequencyof oscillation, and the effect of changing the resistance of one or bothof the resistors 208 and 209 and also changing one or more of theresistors of the resistance network 210 is to effect a multiplication ofthe changes. For example, if the effect of a change in the resistor 208were to increase the frequency of oscillation by a factor of 3.0 and theeffect of a change in the resistor 216 were to change the frequency ofoscillation by a factor of 0.9, the net effect of changing both theresistor 208 and the resistor 216 would be to increase the frequency bya factor of 2.7. The foregoing is true so long as the impedence atoperating frequencies of the components comprising the coils 211 and 222and the resistors 208 and 209 is large as compared with the impedence ofthe resistance network 210. A mathematical evaluation of the operationof the oscillator circuit is as follows:

The oscillator output frequency is a function of the resistors and thecoils which tune the negative feedback loop. While the coils 211 and 222could be adjusted to vary the frequency, it is much more convenient toadjust the resistors. The frequency of the oscillator is given by theequation:

The numeral suffixes in Equations (1), (2) and (3) correspond to thereference numerals of the circuit elements shown in FIG. 9. For example,R in Equation (1 is the resistance of resistor 208 in FIG. 9.

The equation for the mass flow rate of the air through a venturi is:

M ir

a y abs J? where K 1 is a constant; P,,,,, is the absolute or barometricpressure; A P is the pressure drop in the venturi; and T is the absolutetemperature of the air. Similarly, the mass flow rate of the fuelthrough the orifice is:

K'JAP M ue a. M

where K is a constant;

A P is the pressure drop through the orifice; and f (T is a function ofthe absolute temperature and the flow characteristics of the fuel.

It will be apparent from Equation (1) that the variables are square rootfunctions and that changes in any two or more of the variables willeffect a multiplication of the changes. All of the variables inEquations (4) and (5), with the exception of flT are clearly square rootfunctions. In practice, f( T,,,,, is also in effect a square rootfunction because the fuel temperature sensor 42 is designed to match aparticular fuel. Mass flow of a fuel is a function of its viscosity anddensity which of course vary with temperature, and the sensor 42 isdesigned with a temperature to resistance characteristic which producesa nearly square root variation for a particular fuel.

For example, when the mass flow rate of air is being measured, thepotentiometer of the barometer 34 (FIG. 2) corresponds to the resistor209 (FIG. 9), the potentiometer of the bellows sensor 35 corresponds tothe resistor 208,. and the temperature sensor 33 corresponds to theresistor 216. In such a circuit arrangement, the resistors 213 to 215may also be made varia ble so that standardization between measuringsystems may be obtained.

When the mass flow rate of fuel is being measured, the potentiometer 114connected to the bellows 111 corresponds to the resistor 208, forexample, and the temperature sensor 42 corresponds to the resistor 209.If desired, a device may be provided to sense the viscosity of the fuel,such a device including a potentiometer connected as the resistor 216.

To obtain a linear relation between frequency and the mass flow beingsensed, it is important that the variable resistance of the sensors forthe parameters be varied linearly with changes in the magnitudes of theparameters.

FIG. 10 illustrates a circuit for sensing the fuel level in a cup-shapedmetal float bowl 236 of the carburetor. The float bowl 236 has an outlettube 237 connected thereto, the inner end of the tube 237 extending intothe interior of the bowl 236 below the level 238 of the fuel 239 in thebowl 236. The tube 237 slants upwardly and terminates within thecarburetor venturi in the conventional manner. A conventional fuelsupply line and valve (not shown) is also provided for feeding fuel tothe bowl 236. A float 240 controls operation of the valve. The fuellevel sensing circuit comprises an electrical heating coil 241preferably wound around a heatconducting metal core 242 which has apointed end positioned against the outer wall surface of the float bowl236. Two heat-sensitive electrical elements 243 and 244 are positionedabove and below the heating coil 51. The two temperature-sensitiveelements 243 and 244 may comprise thermistors which are connected in anoscillator circuit of the character shown in FIG. 9. In the portion ofthe oscillator circuit shownin FIG. 10, the thermistors 243 and 244 arerespectively equivalent to the resistors 215 and 216 (FIG. 9) of theoscillator circuit. Thus, variations in the resistances of the twothermistors 243 and 244 will effect a change in the frequency ofoscillation in the oscillator circuit.

When the heating coil 241 is energized, the wall of the metal float bowl236 adjacent the metal core 242 will be heated and the wall will conductheat from the area of the core 242. Assuming that the liquid level 238is approximately at the level of the core 242, the thermistor 243 willsense a higher temperature than the thermistor 244 because the liquid239 opposite the thermistor 244 will conduct heat more easily than theair opposite the thermistor 243. If the level 238 of the liquid were ata lower point, for example, below the level of the thermistor 244, thewalls adjacent the two thermistors 243 and 244 would be at approximatelythe same temperature, and this temperature would be relatively high. Ifthe level 238 were above the level of the element 243, the resistancesof the two elements 243 and 244 would again be approximately equal butthe temperature would be relatively low. It will be apparent, therefore,that with the connections as shown in FIG. 10, the frequency will peakwhen the core 242 is adjacent the level 238 and the temperaturedifference is greatest between the thermistors 243 and 244.

Instead of connecting both of the thermistors 243 and 244 in a singleoscillator circuit as shown in FIG. 9, the two thermistors could beconnected as control resistors, such as the resistor 213, in separateoscillator circuits such as the oscillators of the transducers 38 and asshown in FIG. 2. If the other parameters were held constant, thedifference in the frequencies of the two oscillators would beproportional tothe difference in. temperatures of the two thermistors.Since the greatest temperature difference would occur when the heatingcoil is at the level 238 of the fuel 239, the fuel level could bedetermined by noting the position of the probe at which the greatestfrequency difference occurs. As will be explained in connection withFIG. 11, the system may be programmed to compute the ratio of the twofrequencies, and this ratio would be greatest when the probe is adjacentthe fuel level.

FIG. 11 illustrates in greater detail the control unit 66, the counter68, the display-printer 71, and the range selector and control 73, whichreceive the frequency modulated signals from the transducers, computethe desired information, and provide a visual display of the computedinformation and/or a printed record of the information. The structureshown in FIG. 11 may include manual controls and/or a card reader, forexample. In the present construction, both are provided. Further, thestructure provides both a visual display and a printed record.

The structure illustrated in FIG. 11 includes a conventional card reader251 which, for example, has six signal inputs 252 through 257. In thepresent instance the input 252 is connected to receive the signal fromthe transducer 38 representing air flow rate, the input 253 is connectedto receive the signal from the transducer 44 representing fuel flowrate, the input 254 is connected to receive the signal from anoscillator and the fuel level sensing probe of the character shown inFIG. 10, the input 255 may be connected to receive the output of thecoil 59 representing engine speed, and the inputs 256 and 257 may beconnected to receive any other parameters being sensed, such asacceleration and ambient temperature. The card reader 251 receives aprepunched card such as the card 67 shown in FIG. 2, and, depending uponthe arrangement of the punched information, programs the remainder ofthe components for operation in a required mode.

Assuming that a card has been inserted into the card reader 251, whichis punched to obtain a reading of airto-fuel ratio, the card reader 251directs the air flow rate signal appearing on the input 252 to apre-amplifier 261 and to a routing control switch 262. The fuel flowrate signal appearing on the input 253 is directed to a pre-amplifier263 and to another input of the routing control switch 262. The cardreader 251 also provides a mode select output 266 which sets a modecontrol circuit 267 to route the signals from the pre-amplifiers 261 and263 to two gate switches 268 and 269,

this being accomplished by a signal received on a conductor 271 by theswitch 262 from the mode control 267. The mode control 267 also includesa manually operable mode select control 272 which may be set in theevent the card reader is not used. The mode control 267 also signals acycle control 273 to begin a computing cycle. At the beginning of eachcomputing cycle, the cycle control 273 generates a preset signal on aconductor 274 which presets a count register 276 to zero or to a presetnumber. The register 276 includes, in the present example, five decadecounters. In certain operations, the count register 276 is preset to agiven number by a signal from a coefficient switch 279 which in turnreceives directions on a conductor 281 from the card reader 251 or froma manually operable control 282. The coefficient switch 279 is operableto preset the count register 276 at the beginning of each cycle to agiven number, the number depending ,upon the setting of the switch 279,and the count register 276 counts upwardly from the preset number, suchoperation being useful when a curve of the parameter being counted doesnot start at zero. The cycle control 273 also generates a preset signalon a conductor 277 which is received by a gating interval register 278having, for example, five registers. The gating interval register 278also receives an input signal from a coefficient switch 283 which may bemanually set by a control 284 or automatically set by a gating intervalselect output 286 from the card reader 251. The coefficient switch 283is operable to preset the register 278 at the beginning of eachcomputing cycle to a predetermined number, this number representing thetime interval for the counting operation. In the present instance, theregister 278 is preset to a number below zero, and the register 278 ineach computing cycle counts upwardly the number of pulses from the gateswitch 269 until the register 278 reaches zero.

Briefly, the operation of the foregoing portion of the computer in eachcycle is as follows: after a card is inserted into the card reader 251,selected signals, such as the signals on the inputs 252 and 253, arepassed to the preamplifiers 261 and 263 and to the routing con- .trolswitch 262. The coefficient switches are also set by the signals fromthe card reader 251. The mode control 267 actuates the cycle control 273to begin a computing cycle, and at the beginning of each cycle the cyclecontrol 273 presets the count register 276 and the gating intervalregister 278 to the values to which the coefficient switches 279 and 283are set. The cycle control 273 then generates signals on two gatecontrol inputs 287 and 288 of the gate switches 268 and 269,respectively. These signals open the gate switches 268 and 269 and thuspass the air flow rate and the fuel flow rate signals to the tworegisters 276 and 278. The count register 276 then begins countingpulses upwardly from its preset number, and the gating interval register278 begins counting toward zero from its preset number. When the gatinginterval register 278 reaches zero count, it generates a signal on aconductor 291 which is received by the cycle control 273 causing it toclose the two gate switches 268 and 269. The count contained in thecount register 276 is, of course, the preset number plus the number ofair flow rate pulses received during the time required for the gatinginterval register 278 to receive the preset number of fuel flow ratepulses.

The count contained in the count register 276 is fed to a five-digitvisual display 292 which may comprise Nixie tubes. A storage circuit 293is preferably connected between the register 276 and the display 292which, when manually set, will cause the display to hold a count until asubsequent count is to be displayed. If the storage circuit 293 is notmanually actuated, the display 292 will indicate the count contained atany instant in the count register 276.

The length of the display time may, if desired, be manually adjusted byan input 294 to the cycle control 273, the input 294 adjusting the timebetween successive cycles initiated by the cycle control 273.

As previously mentioned, it is preferred that a printed record be madeof computed data, and to this end there is provided a five-digit printer296 which has its input connected to an output of the count register276. The printer 296 also has a zero or reset input connected to thecycle control so that the printer 296 may be reset to zero. The printer296 may, for example, be an electronic printer or a ratchet operatedmechanical printer. Where it is a mechanical printer, the cycle control273 also includes a l-pulse generator 298 which, on receipt of a printcommand signal, transfers a series of pulses to both the count register276 and to the printer 296. The printer 296, in response to the 10 pulseseries, receives the count on the register 276 and prints it, in anoperation known to those skilled in the art. While the foregoingoperation has been described in connection with a mechanical printingstructure, it should be understood that a completely electronic printingapparatus could be employed.

When evaluating the operation of a carburetor, it is desirable to beable to take spot readings or measurements at different air flow rates.In the present example, measurements are taken at five different flowrates. For a particular reading, the switch 186 of the multiple venturistructure 31 is set to connect a venturi to the transducer 35, theconnected venturi being proper for the air flow range in which the spotreading is to be taken. In each range, the amount of air drawn throughthe carburetor varies and consequently the frequency of the signalappearing at the output of the preamplifier 261 also varies. Thepreamplifier 261 output, which indicates the mass flow rate of air, isalso connected by a conductor 301 to the input of a range circuit 302.In the present construction, it is contemplated that a different card beinserted into the card reader 251 for each spot reading and, for eachcard, a range output 303 of the card reader 251 generates a signal whichis received by the range circuit 302 on a conductor 304. The circuit 302includes two frequency responsive electronic switches 306 and 307 whichare connected to receive the frequency modulated signal representing'the mass flow rate of air. As previously mentioned, a

separate card is used in the present example for each reading, and eachcard programs the system to take a reading in a certain range. Inaddition to the signal on the conductor 304, a signal is also providedon an output 305 of the reader 251 which is connected to the switch 186(FIG. 6) in order to connect the proper venturi required in the air flowrange in which a reading is to be taken. The signal appearing on theconductor 304 sets the switch 306 so that it switches at the low end ofthe range, and the switch 307 so that it switches at the high end of therange. When the frequency of the air flow rate signal is within therange, the switches 306 and 307 actuate a logic circuit 308. Prior tothis time, the logic circuit 308 delivers a print inhibit signal, butwhen the frequency of the air flow signal is in the designated range,the inhibit signal is removed and the cycle control orders a printoperation. Further, if the frequency of the signal on the conductor 301is below the designated range, the switch 306 energizes an indicatorlight 311, and if the frequency is above the range, the switch 307energizes a light 312. In addition, the circuit 302 may also include aplurality of manual inputs indicated generally by the numeral 313 whichmay be used in place of the card reader 251.

If desired, the cycle control 273 may also be provided with atwo-position manually operable print switch 316. The switch 316 may beset in an on position where the system will automatically perform aprinting operation at the end of each computing cycle, or in an offposition where the system will perform a printing operation only whenthe switch 316 is momentarily turned on. The cycle control 273 may alsoin clude a manually operable reset control 317 which may be used toinitiate a new computing cycle at any desired time.

While the card reader 251 has been described as requiring a separatecard for each speed range and a separate card for each computing mode ormeasurement to be taken, it should be understood that a different typeof card reader and card could be employed which may be programmed toconsecutively compute in different modes for each speed range, and toinitiate computing cycles automatically in various speed ranges.

While the circuit 302 has been described in an operation where itinhibits a print signal except when the engine speed is within apredetermined range, it should be understood that such a circuit is alsouseful for indicating error in readings where a given frequency isoutside of a specified range of frequencies.

The structure shown in FIG. 11 has been described above in operation inthe ratio mode" wherein the ratio of the air and fuel mass flow ratesignals is obtained. If operation is required in a mode for measuring aparameter such as engine speed, the switch routes the signal from anoscillator 264 to the register 278 and the speed representative signalto the register 276. The frequency of the oscillator 264 is fixed at,for example, KHZ, and is used as a time reference signal. Since thefrequency of the speed signal varies linearly with speed, operation ofthe computer-counter can be described by the equation:

Output=mf b (6) where m is the gate time of register 278; f, is thefrequency of the signal into the register 276; and b is the count whichis preset into the register 276.

When the system is operating in the ratio mode, as when obtaining theair-to-fuel ratio, it operates according to the equation:

OUTPUT =(mf1/f2)+ b (7) where f is the frequency of the signal into theregister 278 and the other quantities are the same as in Equation (6).

From the foregoing it will be apparent that a novel and useful systemhas been provided. The system is capable of sensing different parametersof a carburetor under a wide range of operating conditions, andproviding visual and printed indications of the desired information. Thesystem further includes a novel oscillator circuit which responds to oneor more parameters, a novel air flow rate measuring structure, a novelfuel flow rate measuring structure, and a novel fuel level sensor.

We claim:

1. A system for evaluating the operation of a carburetor, comprisingfirst means adapted to be connected to the air intake of the carburetorand responding to the velocity of air flow to said carburetor, secondmeans adapted to be connected to said air intake and responding to thetemperature of said air, third means adapted to respond to absolute airpressure, a first signal generator connected to said first, second andthird means and producing a first variable frequency signal representingthe mass of air per unit of time flowing into the carburetor, fourthmeans adapted to be connected to the fuel intake of the carburetor andresponding to the velocity of fuel flow to said carburetor, fifth meansadapted to be connected to said fuel intake and responding to thetemperature of said fuel, a second signal generator connected to saidfourth and fifth means and producing a second variable frequency signalrepresenting the mass of fuel per unit of time flowing into thecarburetor, and computer means connected to receive said first andsecond signals and to compute the ratio thereof.

2. A system as in claim 1, wherein said computing means comprises arange select circuit connected to receive said signal from said firsttransducer, means connected to receive said computed ratio and make arecording thereof, and said range circuit being connected to controloperation of said recording means and to enable operation of saidrecording means only when the mass flow rate of air is in apredetermined range.

3. A system as in claim 1, wherein said computing means includes readermeans adapted to receive recorded instructions, said reader meanscontrolling the operation of said first transducer means and of saidcomputing means in a range of air flow in accordance with the recordedinstructions.

4. A system as in claim 1, wherein said first means includes an area ofknown size through which the air flows, and said fourth means includesan area of known size through which the fuel flows.

5. A system as in claim 4, wherein said areas are fixed.

6. A system for evaluating the operation of a carburetor under normaloperating conditions, comprising first transducer means adapted to beconnected to the air intake of the carburetor and generating anelectrical signal having a characteristic which is a function of themass of air per unit of time flowing into the carburetor, secondtransducer means adapted to be connected to the fuel intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of fuel per unit of time flowing intothe carburetor, and computer means connected to receive said signals andto compute the ratio thereof, each of said transducers including aresistance-to-frequency oscillator, said oscillator comprising a notchtuning circuit connected in a feedback loop thereof, said notch tuningcircuit comprising an input terminal and an output terminal, a firstresistor, a second resistor, a resistance network, said first and secondresistors and said network being in series between said input and outputterminals, a first inductor connected to the juncture of said first andsecond resistors, and a second inductor connecting said output terminalwith said resistance network, the frequency to which said notch tuningcircuit is tuned being the frequency of oscillation of said oscillator,whereby a variation in one of said first and second resistances and avariation of a resistor of said resistance networkeffecting a change insaid frequency which is a multiple of the respective change in theresistance.

7. A system as in claim 6, wherein said resistance network comprises twopairs of resistors, said two pairs being connected in parallel andbetween said input terminal and said second resistor, said secondinductor being connected to the juncture of the resistors of one of saidpairs, and said first inductor being connected to the juncture of theresistors of the other of said pairs.

8. A system for evaluating the operation of a carburetor under normaloperating conditions, comprising first transducer means adapted to beconnected to the air intake of the carburetor and generating anelectrical signal having a characteristic which is a function of themass of air per unit of time flowing into the carburetor, secondtransducer means adapted to be connected to the, fuel intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of fuel per unit of time flowing intothe carburetor, computer means connected to receive said signals and tocompute the ratio thereof, said first transducer means comprising arelatively large venturi and a relatively small venturi, said relativelysmall venturi being mounted concentrically within said relatively largeventuri and the downstream end of said relatively small venturiterminating approximately in the throat of said relatively largeventuri, and means for sensing the difference in pressures between theentrance to said venturis and the throat of each of said venturis, andfurther including means for sensing the temperature of the air flowingthrough said venturis, means for sensing barometric pressure, and avariable frequency oscillator connected to said pressure differencesensing means and to said means for sensing temperature and to saidmeans for sensing barometric pressure, said oscillator providing asignal having a frequency representing the mass flow rate of air flowingthrough said venturis.

9. A system for evaluating the operation of a carburetor under normaloperating conditions, comprising first transducer means adapted to beconnected to the air intake of the carburetor and generating anelectrical signal having a characteristic which is a function of themass of air per unit of time flowing into the carburetor, secondtransducer means adapted to be connected to the fuel intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of fuel per unit of time flowing intothe carburetor, and computer means connected to receive said signals andto compute the ratio thereof, said second transducer means comprisingfirst and second enclosures, an orifice connecting said first enclosurewith said second enclosure, means for conducting fuel to said firstenclosure and away from said second enclosure, said fuel flowing fromsaid first enclosure through said orifice to said second enclosure, andpressure difference sensing means connected to sense the pressure insaid first and second enclosures and providing an output representingthe pressure difference between the first and second enclosures, andfurther including a second orifice connecting said first and secondenclosures, and a relief valve movable between a completely closedposition and a completely open position, said valve shifting from saidclosed position to said open position at a predetermined pressuredifference level, said valve sealing said second orifice only atpressure differences below said predetermined level.

10. A system as in claim 9, wherein said second orifice is larger thansaid first orifice.

1 l. A system for evaluating the operation of a carburetor under normaloperating conditions, comprising first transducer means adapted to beconnected to the air intake of the carburetor and generating anelectrical signal having a characteristic which is a function of themass of air per unit of time flowing into the carburetor, secondtransducer means adapted to be connected to the fuel intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of fuel per unit of time flowing intothe carburetor, and computer means connected to receive said signals andto compute the ratio thereof, said second transducer means comprisingfirst and second enclosures, an orifice connecting said first enclosurewith said second enclosure, means for conducting fuel to said firstenclosure and away from said second enclosure, said fuel flowing fromsaid first enclosure through said orifice to said second enclosure, andpressure difference sensing means connected to sense the pressure insaid first and second enclosures and providing an output representingthe pressure difference between the first and second enclosures, saidsecond transducer means further including a second orifice connectingsaid first and second enclosures, and a relief valve sealing said secondorifice only at pressure difference below a predetermined level, saidrelief valve comprising a seal, magnet means for urging said seal to aposition where it closes said second orifice and tending to hold saidseal in said position, and a compression spring positioned to urge saidseal away from said second orifice, whereby said compression spring andthe fuel pressure acting through said second orifice and against saidseal combine at said predetermined pressure difference to overcome theforce of said magnet.

12. A system for evaluating the operation of a carburetor under normaloperating conditions, comprising first transducer means adapted to beconnected to the air intake of the carburetor and generating anelectrical signal having a characteristic which is a function of themass of air per unit of time flowing into the carburetor, secondtransducer means adapted to be connected to the fuel intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of fuel per unit of time flowing intothe carburetor, and computer means connected to receive aid si s an toco ute th rati ereof, said arbureto r iilcluding a tl t bow, and urtherincluding means adapted to sense the level of fuel in said float bowl,said fuel level sensing means comprising a heating element adapted to bepositioned against an exterior wall of the float bowl adjacent the levelof the fuel within the float bowl, first and second vertically spacedtemperature sensitive electrical resistors positioned adjacent saidheating element, said first resistor being positioned above said heatingelement and said second resistor being positioned below said heatingelement, and at least one variable frequency oscillator circuitincluding said first and second resistors, the frequency of oscillationof said oscillator circuit being a function of the difference intemperatures of said first and second resistors.

13. A system as in claim 12, and further including a second variablefrequency oscillator circuit, one of said temperature sensitiveresistors being connected as an element in one of said oscillatorcircuits and the other of said temperature sensitive resistors beingconnected as an element in the other of said oscillators, said first andsecond resistors affecting the frequencies of said oscillators, andmeans connected to the outputs of said two oscillators for comparing thefrequencies of oscillation thereof.

14. A system for evaluating the operation of a carburetor under normaloperating conditions, comprising first transducer means adapted to beconnected to the air intake of the carburetor and generating anelectrical signal having a characteristic which is a function of themass of air per unit of time flowing into the carburetor, secondtransducer means adapted to be connected to the fuel intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of fuel per unit of time flowing intothe carburetor, and computer means connected to receive said signals andto compute the ratio thereof, said characteristics of said first andsecond signals comprising the frequencies thereof, said frequenciesvarying substantially linearly with changes in the air and fuel massflow rates, and said computing means utilizing said signal of saidsecond transducer for forming a time base signal, and counting meansconnected to receive the signal of said first transducer and countingthe number of pulses therein occurring during the interval of said timebase signal.

1. A system for evaluating the operation of a carburetor, comprisingfirst means adapted to be connected to the air intake of the carburetorand responding to the velocity of air flow to said carburetor, secondmeans adapted to be connected to said air intake and responding to thetemperature of said air, third means adapted to respond to absolute airpressure, a first signal generator connected to said first, second andthird means and producing a first variable frequency signal representingthe mass of air per unit of time flowing into the carburetor, fourthmeans adapted to be connected to the fuel intake of the carburetor andresponding to the velocity of fuel flow to said carburetor, fifth meansadapted to be connected to said fuel intake and responding to thetemperature of said fuel, a second signal generator connected to saidfourth and fifth means and producing a second variable frequency signalrepresenting the mass of fuel per unit of time flowing into thecarburetor, and computer means connected to receive said first andsecond signals and to compute the ratio thereof.
 2. A system as in claim1, wherein said computing means comprises a range select circuitconnected to receive said signal from said first transducer, meansconnected to receive said computed ratio and make a recording thereof,and said range circuit being connected to control operation of saidrecording means and to enable operation of said recording means onlywhen the mass flow rate of air is in a predetermined range.
 3. A systemas in claim 1, wherein said computing means includes reader meansadapted to receive recorded instructions, said reader means controllingthe operation of said first transducer means and of said computing meansin a range of air flow in accordance with the recorded instructions. 4.A system as in claim 1, wherein said first means includes an area ofknown size through which the air flows, and said fourth means includesan area of known size through which the fuel flows.
 5. A system as inclaim 4, wherein said areas are fixed.
 6. A system for evaluating theoperation of a carburetor under normal operating conditions, comprisingfirst transducer means adapted to be connected to the air intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of air per unit of time flowing into thecarburetor, second transducer means adapted to be connected to the fuelintake of the carburetor and generating an electrical signal having acharacteristic which is a function of the mass of fuel per unit of timeflowing into the carburetor, and computer means connected to receivesaid signals and to compute the ratio thereof, each of said transducersincluding a resistance-to-frequency oscillator, said oscillatorcomprising a notch tuning circuit connected in a feedback loop thereof,said notch tuning circuit comprising an input terminal and an outputterminal, a first resistor, a second resistor, a resistance network,said first And second resistors and said network being in series betweensaid input and output terminals, a first inductor connected to thejuncture of said first and second resistors, and a second inductorconnecting said output terminal with said resistance network, thefrequency to which said notch tuning circuit is tuned being thefrequency of oscillation of said oscillator, whereby a variation in oneof said first and second resistances and a variation of a resistor ofsaid resistance network effecting a change in said frequency which is amultiple of the respective change in the resistance.
 7. A system as inclaim 6, wherein said resistance network comprises two pairs ofresistors, said two pairs being connected in parallel and between saidinput terminal and said second resistor, said second inductor beingconnected to the juncture of the resistors of one of said pairs, andsaid first inductor being connected to the juncture of the resistors ofthe other of said pairs.
 8. A system for evaluating the operation of acarburetor under normal operating conditions, comprising firsttransducer means adapted to be connected to the air intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of air per unit of time flowing into thecarburetor, second transducer means adapted to be connected to the fuelintake of the carburetor and generating an electrical signal having acharacteristic which is a function of the mass of fuel per unit of timeflowing into the carburetor, computer means connected to receive saidsignals and to compute the ratio thereof, said first transducer meanscomprising a relatively large venturi and a relatively small venturi,said relatively small venturi being mounted concentrically within saidrelatively large venturi and the downstream end of said relatively smallventuri terminating approximately in the throat of said relatively largeventuri, and means for sensing the difference in pressures between theentrance to said venturis and the throat of each of said venturis, andfurther including means for sensing the temperature of the air flowingthrough said venturis, means for sensing barometric pressure, and avariable frequency oscillator connected to said pressure differencesensing means and to said means for sensing temperature and to saidmeans for sensing barometric pressure, said oscillator providing asignal having a frequency representing the mass flow rate of air flowingthrough said venturis.
 9. A system for evaluating the operation of acarburetor under normal operating conditions, comprising firsttransducer means adapted to be connected to the air intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of air per unit of time flowing into thecarburetor, second transducer means adapted to be connected to the fuelintake of the carburetor and generating an electrical signal having acharacteristic which is a function of the mass of fuel per unit of timeflowing into the carburetor, and computer means connected to receivesaid signals and to compute the ratio thereof, said second transducermeans comprising first and second enclosures, an orifice connecting saidfirst enclosure with said second enclosure, means for conducting fuel tosaid first enclosure and away from said second enclosure, said fuelflowing from said first enclosure through said orifice to said secondenclosure, and pressure difference sensing means connected to sense thepressure in said first and second enclosures and providing an outputrepresenting the pressure difference between the first and secondenclosures, and further including a second orifice connecting said firstand second enclosures, and a relief valve movable between a completelyclosed position and a completely open position, said valve shifting fromsaid closed position to said open position at a predetermined pressuredifference level, said valve sealing said second orifice only atpressure diFferences below said predetermined level.
 10. A system as inclaim 9, wherein said second orifice is larger than said first orifice.11. A system for evaluating the operation of a carburetor under normaloperating conditions, comprising first transducer means adapted to beconnected to the air intake of the carburetor and generating anelectrical signal having a characteristic which is a function of themass of air per unit of time flowing into the carburetor, secondtransducer means adapted to be connected to the fuel intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of fuel per unit of time flowing intothe carburetor, and computer means connected to receive said signals andto compute the ratio thereof, said second transducer means comprisingfirst and second enclosures, an orifice connecting said first enclosurewith said second enclosure, means for conducting fuel to said firstenclosure and away from said second enclosure, said fuel flowing fromsaid first enclosure through said orifice to said second enclosure, andpressure difference sensing means connected to sense the pressure insaid first and second enclosures and providing an output representingthe pressure difference between the first and second enclosures, saidsecond transducer means further including a second orifice connectingsaid first and second enclosures, and a relief valve sealing said secondorifice only at pressure difference below a predetermined level, saidrelief valve comprising a seal, magnet means for urging said seal to aposition where it closes said second orifice and tending to hold saidseal in said position, and a compression spring positioned to urge saidseal away from said second orifice, whereby said compression spring andthe fuel pressure acting through said second orifice and against saidseal combine at said predetermined pressure difference to overcome theforce of said magnet.
 12. A system for evaluating the operation of acarburetor under normal operating conditions, comprising firsttransducer means adapted to be connected to the air intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of air per unit of time flowing into thecarburetor, second transducer means adapted to be connected to the fuelintake of the carburetor and generating an electrical signal having acharacteristic which is a function of the mass of fuel per unit of timeflowing into the carburetor, and computer means connected to receivesaid signals and to compute the ratio thereof, said carburetor includinga float bowl, and further including means adapted to sense the level offuel in said float bowl, said fuel level sensing means comprising aheating element adapted to be positioned against an exterior wall of thefloat bowl adjacent the level of the fuel within the float bowl, firstand second vertically spaced temperature sensitive electrical resistorspositioned adjacent said heating element, said first resistor beingpositioned above said heating element and said second resistor beingpositioned below said heating element, and at least one variablefrequency oscillator circuit including said first and second resistors,the frequency of oscillation of said oscillator circuit being a functionof the difference in temperatures of said first and second resistors.13. A system as in claim 12, and further including a second variablefrequency oscillator circuit, one of said temperature sensitiveresistors being connected as an element in one of said oscillatorcircuits and the other of said temperature sensitive resistors beingconnected as an element in the other of said oscillators, said first andsecond resistors affecting the frequencies of said oscillators, andmeans connected to the outputs of said two oscillators for comparing thefrequencies of oscillation thereof.
 14. A system for evaluating theoperation of a carburetor under normal operating condItions, comprisingfirst transducer means adapted to be connected to the air intake of thecarburetor and generating an electrical signal having a characteristicwhich is a function of the mass of air per unit of time flowing into thecarburetor, second transducer means adapted to be connected to the fuelintake of the carburetor and generating an electrical signal having acharacteristic which is a function of the mass of fuel per unit of timeflowing into the carburetor, and computer means connected to receivesaid signals and to compute the ratio thereof, said characteristics ofsaid first and second signals comprising the frequencies thereof, saidfrequencies varying substantially linearly with changes in the air andfuel mass flow rates, and said computing means utilizing said signal ofsaid second transducer for forming a time base signal, and countingmeans connected to receive the signal of said first transducer andcounting the number of pulses therein occurring during the interval ofsaid time base signal.